Idle mode handling in a hybrid GSM/CDMA network

ABSTRACT

In a mobile wireless telecommunications system, which includes base stations ( 30 ) of a first type operating over a first air interface, and base stations ( 32 ) of a second type operating over a second air interface, a method for reselection by a mobile station ( 40 ) camped on a cell associated with a first base station ( 30 ), which is of the first type, of a second base station ( 32 ), which is of the second type. The method includes receiving signals over the second air interface from the second base station ( 32 ) and evaluating a characteristic of the signals. Responsive to the characteristic, the second base station ( 32 ) is selected in place of the first base station ( 30 ), and the mobile station ( 40 ) camps on a cell associated with the second base station ( 32 ).

FIELD OF THE INVENTION

The present invention relates generally to wireless telecommunications,and specifically to advanced cellular telephone networks.

BACKGROUND OF THE INVENTION

The Global System for Mobile (GSM) telecommunications is used incellular telephone networks in many countries around the world. GSMoffers a useful range of network services and standards. Existing GSMnetworks are based on time-division multiple access (TDMA) digitalcommunications technology, but an evolution is taking place toward theuse of code-division multiple access (CDMA) technology. The presentinvention is applicable to both existing and future GSM networks, andalthough terminology used in the present patent application relates toexisting GSM standards, it will be understood that the present inventionis in no way limited to currently-existing standards or networks.

Handover in cellular systems can take place either while the MS isoperating in a dedicated mode, i.e., during a telephone call, or whilethe MS is in an idle mode, between calls. The objective of the idle modehandover is to have the MS “camped on” the most appropriate base stationcell of the network (i.e., tuned to the cell's control channels in orderto receive paging and broadcast signals therefrom), and registered inthe network via that cell. The MS is then ready to initiate or receiveservice requests immediately upon demand.

The GSM family of standards, and in particular, GSM standard 03.22,which is incorporated herein by reference, define idle mode operation interms of three major, interrelated processes:

-   -   Selection of a public land mobile network (PLMN);    -   Cell selection and reselection; and    -   Location updating.        In the protocol stack defined by GSM standards for signaling        between the MS and base station, these idle mode functions are        carried out by a radio interface protocol layer 3 (RIL-3).        Within this layer, the cell selection/reselection process is        carried out by a radio resource management (RR) sub-layer; and        the PLMN selection and location updating processes are carried        out by a mobility management (MM) sub-layer.

Every time the MS enters the idle mode (upon switch-on or upontermination of a call), it selects a PLMN and attempts to camp on a cellof that PLMN. The particular PLMN to be contacted may be selected eithermanually or automatically, according to a predefined order ofpriorities. The cell selection procedure verifies that the MS is campedon a suitable cell, in which it will be able to reliably receive anddecode data and in which it is likely to be granted access wheninitiating a call. Cell selection may be based on a general search ofall possible base station channels, or it may be aided by reference to astored list of cells and/or cell signal strength measurements made whilethe MS was in dedicated mode. The alternative methods of cell selectionare referred to in GSM parlance as follows:

-   -   Normal Cell Selection, wherein the MS searches all channels in        all supported bands of operation;    -   Stored List Cell Selection, wherein the search is based on a        stored list of cells in order to speed up the initial cell        selection; and    -   Choose Cell Selection wherein the MS uses measurements made in        dedicated mode in order to speed up cell selection after call        termination.        Once the MS has selected and camped on the appropriate cell, it        sends a location update message to the base station so as to        register its location with the network.

The MS continually monitors the signal received from the current cell onwhich it is camped, as well as the signals from neighboring cells. If acell change is indicated, for example, because the signal from one ofthe neighboring cells is stronger than that of the current cell, orbecause of network conditions and priorities, cell reselection isinvoked, followed by location update as required. If the MS losescontact with the current cell, cell selection and, if necessary, PLMNselection are likewise invoked.

If no suitable cell is found or the MS is not permitted to receiveservice (if, for example, the GSM subscriber identity module [SIM] isnot properly inserted, or the network rejects a location updaterequest), the MS enters a limited service mode. In limited service, theMS attempts to camp on any cell that will allow it to make emergencycalls, irrespective of its PLMN identity.

Although the above description refers specifically to GSM standards andTDMA operation, handover and idle mode functions are a part of othercellular systems and standards, as well. Code-division multiple access(CDMA) is an improved digital communications technology, which affordsmore efficient use of radio bandwidth than TDMA, as well as a morereliable, fade-free link between cellular telephone subscribers and basestations. The CDMA standard that is currently deployed is TIA/EIA-95(commonly referred to as IS-95), promulgated by the TelecommunicationsIndustry Association (TIA).

GSM and CDMA standards of relevance to the present patent applicationare listed for reference in Appendix A at the end of this specification.

Hybrid GSM/CDMA cellular communications systems are described in thepatent literature, although none have yet been commercially deployed.For example, PCT patent application PCT/US96/20764, which isincorporated herein by reference, describes a wirelesstelecommunications system that uses a CDMA air interface (i.e., basic RFcommunications protocols) to implement GSM network services andprotocols. Using this system, at least some of the TDMA base stations(BSSs) and subscriber units of an existing GSM network would be replacedor supplemented by corresponding CDMA equipment. CDMA BSSs in thissystem are adapted to communicate with GSM mobile switching centers(MSCs) via a standard GSM A-interface. The core of GSM network servicesis thus maintained, and the changeover from TDMA to CDMA is transparentto users.

Hybrid cellular communications networks, incorporating both GSM and CDMAelements, are also described in PCT patent publications WO 95/24771 andWO 96/21999, and in an article by Tscha, et al., entitled “A SubscriberSignaling Gateway between CDMA Mobile Station and GSM Mobile SwitchingCenter,” in Proceedings of the 2nd International Conference on UniversalPersonal Communications, Ottawa (1993), pp. 181-185, which areincorporated herein by reference. None of these publications deals withspecific issues of how to implement efficient handovers and idle modeoperation in such hybrid networks.

PCT patent application PCT/US97/00926, which is also incorporated hereinby reference, describes methods of dedicated intersystem handoverbetween CDMA and TDMA BSSs in a hybrid GSM/CDMA telecommunicationssystem. A GSM/TDMA BSS generates pilot beacon signals in accordance withCDMA technology. During a telephone call, a subscriber unit detects thepilot signals and notifies a base station controller that the signalshave been detected. The subscriber unit is then handed over from theCDMA to the TDMA BSS without interrupting the call.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide methods andapparatus for use in a hybrid GSM/CDMA cellular communications network.

It is a further object of some aspects of the present invention toprovide methods enabling improved idle mode operation of mobilesstations in a hybrid cellular network.

It is still a further object of some aspects of the present invention toprovide improved methods and apparatus enabling handover of a mobilestation between TDMA and CDMA base stations while the mobile station isin idle mode.

In preferred embodiments of the present invention, a mixed GSM/CDMAcellular communications system includes both TDMA and CDMA base stationsubsystems (BSSs), associated with a public land mobile network (PLMN).The system is preferably adapted to provide both circuit-switched andpacket-switched services. Systems of this type are described generallyin the above-mentioned PCT patent applications, and in U.S. patentapplication Ser. No. 09/365,967, which is assigned to the assignee ofthe present patent application and whose disclosure is incorporatedherein by reference. A mobile station (MS) in the system is capable ofcommunicating with both types of base stations, by appropriatelyswitching between TDMA and CDMA air interfaces, while preferably usingGSM network protocols over both types of interface. While operating inidle mode, the MS automatically selects a base station cell to camp onby receiving and evaluating signals from both TDMA and CDMA cells.

While the MS is camped on a cell of one type (TDMA or CDMA), it monitorscells of both the same type and of the other type. The MS can reselect acell of either type, as appropriate, based on the signals received andon other predetermined criteria, such as network conditions and userpreferences. The criteria and procedures for monitoring and reselectionare preferably chosen so as to minimize power consumption by the MS inthe idle mode. Further preferably, idle mode operation and cellselection/reselection by the MS are governed by a protocol stackincluding a hybrid GSM/CDMA radio resource management (RR) sub-layer.The hybrid RR sub-layer is aware of and chooses between TDMA and CDMAlower (physical) layers.

Preferably, to reduce cost, weight and power consumption, the MS has asingle transceiver, with respective, selectable modes for GSM/TDMA andCDMA use. It will be appreciated, however, that the principles of thepresent invention may similarly be applied using mobile stations andnetworks of other types, for example, using a mobile station havingseparate or only partially integrated TDMA and CDMA transceivers, or inother hybrid networks that are not GSM-compliant.

In preferred embodiments of the present invention, idle mode behavior ofthe MS is based on balancing the following requirements:

-   1. To camp on the best cell available;-   2. To avoid losing paging messages;-   3. To minimize activity in idle mode in order to save battery life.

Preferably, the MS in idle mode continually assesses signal measurementsfrom both the cell on which the MS is camped (referred to herein as theserving cell) and from neighboring cells, and also receives cellbroadcast information from the network in order to select the mostadequate cell available to camp on. Optimal cell selection has a directinfluence on the rate of success in initiating a service call orresponding to a paging request. Timely cell reselection, when aneighboring cell is found to be preferable to the serving cell, isuseful in avoiding abrupt loss of coverage.

Similarly, in order to avoid losing paging messages, it is important tocamp on the best available cell and to avoid “blind” camping on a cellthat has not been evaluated in advance. This need is balanced, however,by loss of paging messages that may occur due to frequent changes oflocation area or air interface reselection.

The battery life depends on the amount of idle background activity.Every cell change and especially every change of location area or airinterface reselection means increased activity, because the MS must readthe new cell parameters and may have to set up a signaling connectionfor location update if the location area has changed. Monitoringmultiple radio bands (such as the 900 and 1800 MHz bands of GSM) anddual air interfaces also means increased activity that will have impacton the battery life. Therefore, in order to save battery life, the MSpreferably minimizes dual-interface activity and performs air interfacereselection only when necessary.

In preferred embodiments of the present invention, the air interface isreselected under two alternative sets of circumstances:

-   1. Forced reselection, which occurs if there is an abrupt end of    coverage of the active air interface (i.e., the air interface of the    cell on which the MS is camped; the other of the two air interfaces    is referred to in this context as the passive air interface). In    such circumstances, the MS has no prior information regarding the    cells of the new air interface, and must handle the change as though    it were making an initial cell selection, as at power-on, for    example.-   2. Ordered reselection, which occurs upon expiration of predefined    threshold conditions, having to do, for example, with the signal    strengths or qualities of cells on the active air interfaces. In    this case, the MS monitors both the active and passive air    interfaces prior to mode reselection, and the mode change is    preferably handled in a manner generally similar to a cell    reselection.

Optionally, in addition to such network-invoked periodic searches, theMS itself operates a mode search timer so as to invoke periodicmonitoring of the passive mode. The timer enables the MS to avoidendless camping on a less-preferred air interface.

Typically, one of the air interfaces, most preferably the CDMA airinterface, is set by the network or by a user of the MS as the preferredair interface, over which the MS chooses to communicate when a choiceexists between camping on a GSM/TDMA or GSM/CDMA cell. The preferred airinterface is recorded in the MS, either in a subscriber identificationmodule (SIM) inserted in the MS, or in a non-volatile memory of the MSitself. The SIM is preferably compatible with GSM standards, butincludes an extended memory segment (file) for storing information, suchas user preferences, relating to CDMA and hybrid operation.

Preferably, when forced reselection of the air interface occurs, the MSfirst attempts to select the last active interface, when known.Otherwise, the MS may select the user's preferred air interface, if sucha preference is defined and programmed in the SIM, or a default airinterface, programmed in the non-volatile memory. If no suitable cell isfound on the selected air interface, the cell selection terminates, andthe MS attempts to select other air interface(s). Preferably, tooptimize selection when the MS has no a priori information on theexistence or precedence of cells on one air interface or another, aninitial decision on the order of interface selection is preferablypreceded by an initial power measurement of signals received over bothair interfaces on their respective frequency bands. Finally, if allavailable interfaces are attempted and no suitable cell is found, themode selection terminates with a failure indication communicated to anupper protocol layer and thence to the user.

Preferably, the MS undertakes ordered reselection when predeterminedreselection criteria have been met. For example, in a preferredembodiment of the present invention, the MS begins periodic monitoringof the passive air interface if all the following conditions are met:

-   1. The network broadcasts an indication to the MS that neighboring    cells operating over the passive air interface are available; and-   2. The signal levels received by the MS from all cells operating    over the as active air interface are below a predetermined threshold    (which may be broadcast over the network) for a given time period T,    wherein preferably T=5 seconds; and-   3. There are fewer than a predetermined number of cells, preferably    less than two such cells, in a list of “Neighbor Candidate” cells in    the active mode for a given time period T.

The MS then decides whether to perform an air interface reselection bymeasuring and comparing the strengths of signals or the relative qualityof signals received from cells over the active and passive airinterfaces. In some preferred embodiments of the present invention, forpurposes of the decision on air interface reselection, the measuredsignals strengths or qualities are weighted by combined reselectioncriteria, including:

-   -   Air interface preference (or priority), as described        hereinabove;    -   Interface reselection hysteresis, a factor added to prevent        frequent change between the air interfaces;    -   Strong neighbors (SN) assessment, to take into account a        situation in which the MS is located in a border cell of a given        coverage area, so as to change the active mode before        encountering an abrupt end of coverage.        Other suitable criteria will be apparent to those skilled in the        art.

There is therefore provided, in accordance with a preferred embodimentof the present invention, in a mobile wireless telecommunicationssystem, which includes base stations of a first type operating over afirst air interface, and base stations of a second type operating over asecond air interface, a method for reselection by a mobile stationcamped on a cell associated with a first base station, which is of thefirst type, of a second base station, which is of the second type,including:

receiving signals over the second air interface from the second basestation;

evaluating a characteristic of the signals;

responsive to the characteristic, selecting the second base station inplace of the first base station; and

camping on a cell associated with the second base station.

Preferably, one of the first and second air interfaces includes a TDMAair interface, and the other includes a CDMA air interface, whereinevaluating the characteristic includes applying a CDMA path losscriterion to the signals. Additionally or alternatively, selecting thesecond base station includes applying cell selection and reselectionprocedures over the CDMA air interface in a manner substantiallytransparent to a GSM radio interface protocol layer of the mobilestation. Preferably, while the mobile station is camped on the cellassociated with the base station operating over the CDMA air interface,it performs idle mode procedures generally in accordance with a GSMstandard.

Further preferably, selecting the second base station in place of thefirst base station includes using a single radio resource managementprotocol layer in the mobile station supporting both GSM and CDMAoperating modes. In a preferred embodiment, the radio resourcemanagement protocol layer includes parallel GSM and CDMA protocolsublayers and a combiner sublayer which selects either the GSM or theCDMA operating mode. Preferably, the combiner sublayer receives messagesfrom a mobility management protocol layer at a service access point inaccordance with GSM standards, and maps the messages to primitives whichit directs to the selected GSM or CDMA sublayer.

In a preferred embodiment, receiving the signals over the second airinterface includes receiving signals using a single radio transceiver inthe mobile station which is also used to receive the signals over thefirst air interface. Preferably, receiving the signals includesreceiving signals in either a GSM or a CDMA signaling mode.

Preferably, while the mobile station is camped on the cell associatedwith the first base station, it receives signals therefrom duringintermittent active periods of the mobile station, and receiving thesignals over the second air interface includes seeking and receivingsignals during sleep periods of the mobile station intermediate theactive periods.

In a preferred embodiment, receiving the signals includes controllingthe mobile station to receive signals over the second air interfaceresponsive to a detected loss of coverage by signals on the first airinterface.

In another preferred embodiment, receiving the signals includesinitiating monitoring of signals over the second air interfaceresponsive to an indication that a predetermined monitoring criterionhas been met. Preferably, the indication includes a message broadcast tothe mobile station over the first air interface that cells are availableover the second air interface. Further preferably, initiating themonitoring includes initiating monitoring over the second air interfaceresponsive to a level of the signals received over the first airinterface. Most preferably, the mobile station attempts to receivesignals from a plurality of candidate cells over the first airinterface, and wherein initiating the monitoring includes initiatingmonitoring over the second air interface when the signals received overthe first air interface are below a predefined level for a predeterminedperiod of time.

Preferably, the mobile station attempts to receive signals from aplurality of candidate cells over the first air interface, andinitiating the monitoring includes initiating monitoring over the secondair interface when the number of candidate cells over the firstinterface is less than a predetermined minimum number for apredetermined period of time. Additionally or alternatively, initiatingthe monitoring includes initiating monitoring upon expiration of apredetermined time period during which monitoring over the second airinterface has not occurred.

In a preferred embodiment, receiving the signals includes regulatingenergy expended by the mobile station in receiving the signalsresponsive to a desired level of energy consumption by the mobilestation. Preferably, regulating the energy expended includes setting asampling rate at which to receive the signals responsive to the desiredlevel of energy consumption. Additionally or alternatively, regulatingthe energy expended includes choosing a number of the base stations ofthe second type from which to receive the signals responsive to thedesired level of energy consumption. Further additionally oralternatively, regulating the energy expended further includesregulating the availability of the mobile station to receive the signalsresponsive to a desired level of quality of service provided by themobile station.

Preferably, evaluating the characteristic includes comparing the signalsreceived from the second base station to signals received over the firstair interface from the first base station and applying reselectioncriteria to the received signals so as to determine whether to selectthe second base station. In a preferred embodiment, applying thecriteria includes weighting measured characteristics of the signalsresponsive to a predetermined air interface preference. Preferably, thepreference is set by a user of the mobile station. Alternatively, thepreference is set by a network with which the base stations areassociated. Additionally or alternatively, the mobile station stores arecord of the preference.

Preferably, applying the criteria includes applying a predeterminedhysteresis factor so as to prevent recurrent reselection of the airinterface.

In a preferred embodiment, comparing the signals includes performing anassessment of strong neighbor cells when the mobile station is in aborder area of coverage provided over the first air interface.

Preferably, evaluating the characteristic includes comparing powerlevels of the signals received over the first and second air interfacesand comparing path-loss criteria derived from the signals received overthe first and second air interfaces.

Additionally or alternatively, selecting the second base stationincludes selecting a base station responsive to selection by the mobilestation of a public land mobile network with which to communicate.

In a preferred embodiment, selecting the second base station includesreceiving information broadcast over the first air interface in relationto criteria for interface reselection, and selecting the second basestation responsive to the broadcast information.

In another preferred embodiment, selecting the second base stationincludes storing information in a memory module of the mobile station inrelation to criteria for interface reselection, and selecting the secondbase station responsive to the stored information.

There is further provided, in accordance with a preferred embodiment ofthe present invention, in a mobile wireless telecommunications system,which includes a first cell associated with a first air interface, and asecond cell associated with a second air interface, a mobile station,including:

at least one radio transceiver, which receives signals from the firstand second cells over the first and second air interfaces, respectively;and

control circuitry, which processes the signal received from the secondcell while the mobile station is camped in idle mode on the first cell,and which evaluates the second signal and, responsive thereto, directsthe mobile station to reselect and camp on the second cell.

Preferably, the at least one transceiver includes a single radiotransceiver capable of operating over either the first or the second airinterface.

In a preferred embodiment, the mobile station includes a SubscriberInformation Module, which stores information in relation to criteria forinterface reselection, and the control circuitry determines whether themobile station should reselect and camp on the second cell responsive tothe stored information.

There is also provided, in accordance with a preferred embodiment of thepresent invention, in a mobile wireless telecommunications system, amethod for cell reselection by a mobile station camped on a first cell,including:

receiving signals over the air from a second cell;

determining whether the second cell belongs to a different location areafrom the first cell;

evaluating a characteristic of the signals, responsive to the determinedlocation area of the second cell; and

responsive to the evaluation, selecting the second cell for camping inplace of the first cell.

Preferably, evaluating the characteristic of the signals includesapplying a threshold criterion to the signals. The threshold ispreferably relatively higher when the second cell belongs to a differentlocation area than the first cell and is relatively lower when thesecond cell belongs to the same location area as the first cell.

In a preferred embodiment, determining whether the second cell belongsto a different location area includes receiving a broadcast from thefirst cell indicating the location area of the second cell.

In another preferred embodiment, determining whether the second cellbelongs to a different location area includes looking up in a memory ofthe mobile station a stored record of the location area of the secondcell.

There is additionally provided, in accordance with a preferredembodiment of the present invention, in a mobile wirelesstelecommunications system, a mobile station, including:

a radio transceiver, which receives signals from a second cell while themobile station is camped on a first cell; and

control circuitry, which determines whether the second cell belongs to adifferent location area from the first cell and processes the signalsreceived from the second cell responsive to the determined location areaof the second cell, so as to decide whether to select the second cellfor camping in place of the first cell.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS AND APPENDICES

FIG. 1 is a schematic block diagram of a hybrid GSM/CDMA cellularcommunications system, in accordance with a preferred embodiment of thepresent invention;

FIG. 2 is a schematic illustration showing cells in the system of FIG.1, useful in understanding methods of cell and air interface selectionin idle mode, in accordance with preferred embodiments of the presentinvention;

FIG. 3 is a schematic block diagram illustrating communication protocolsbetween a mobile station and base station subsystems in the system ofFIG. 1, in accordance with a preferred embodiment of the presentinvention;

FIG. 4 is a schematic block diagram of a hybrid GSM/CDMA mobile station,in accordance with a preferred embodiment of the present invention;

FIG. 5 is a schematic block diagram illustrating details of protocollayers shown in FIG. 3, in accordance with a preferred embodiment of thepresent invention;

FIG. 6 is a schematic block diagram illustrating message flows betweenthe protocol layers shown in FIG. 3, in accordance with a preferredembodiment of the present invention;

FIGS. 7A-7B are flow charts illustrating idle mode behavior of a mobilestation in the system of FIG. 1, in accordance with a preferredembodiment of the present invention;

FIGS. 8A-8B and 9A-9B are flow charts illustrating idle mode processescarried out by the mobile station, in accordance with a preferredembodiment of the present invention;

FIGS. 10A and 10B are flow charts illustrating methods of idle mode cellselection, in accordance with a preferred embodiment of the presentinvention;

FIG. 11 is a flow chart illustrating a method of idle mode cellreselection, in accordance with a preferred embodiment of the presentinvention;

FIG. 12 is a flow chart illustrating a method of air interfaceselection, in accordance with a preferred embodiment of the presentinvention; and

FIG. 13 is a block diagram that schematically illustrates decisioncriteria for determining whether reselection of the air interface isrequired, in accordance with a preferred embodiment of the presentinvention.

Appendix A contains a listing of published standards of relevance to thepresent invention, which are incorporated herein by reference;

Appendix B contains an explanatory summary of terms and abbreviationsused in the present patent application, provided for the convenience ofthe reader; and

Appendix C contains a description of interfaces, primitives and statevariables associated with radio resource (RR) management protocolscarried out by a mobile station, in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS General Features of aHybrid GSM/CDMA Cellular System

Reference is now made to FIG. 1, which is a schematic block diagram of ahybrid GSM/CDMA cellular communications system 20, in accordance with apreferred embodiment of the present invention. System 20 is built arounda public land mobile network (PLMN) 22, which is based on GSM networkprotocols, as described hereinabove. Although for the sake ofsimplicity, only one PLMN is shown in FIG. 1, there may be multiple,different networks of this sort through which subscriber units cancommunicate.

PLMN 22 comprises at least one mobile-services switching center (MSC)24, or possibly a number of such centers (although only one MSC is shownhere for clarity of illustration), which controls network operationswithin a geographical area. Among other functions, MSC 24 is responsiblefor location registration of subscriber units and handover of subscriberunits between base stations, as well as linking PLMN 22 to a publicswitched telephone network (PSTN) and/or packet data network (PDN) 48.The PLMN also comprises a network management center (NMC) 26 and a cellbroadcast center (CBC) 28, in accordance with GSM standards. Thefunctions and operation of system 20 are described further in theabove-mentioned U.S. patent application Ser. No. 09/365,967.

System 20 includes a plurality of mobile stations (MS) 40, whichcommunicate with PLMN 22 via a plurality of base station subsystems(BSS) 30 and 32 over a wireless RF link in one or more of the acceptedcellular communications frequencies. MS 40, which is also known as asubscriber unit, is capable of communicating with both GSM BSS 30, usinga substantially standard GSM TDMA signaling protocol, and CDMA BSS 32,using CDMA-based communication methods described hereinbelow.

Both GSM BSS 30 and CDMA BSS 32 communicate with and are controlled byMSC 24. Communications between GSM BSS 30 and MSC 24 are substantiallyin accordance with GSM standards. CDMA BSS 32 communicates with PLMN 22in accordance with GSM standards. BSS 32 also communicates with CBC 28,so as to receive messages to be broadcast over the air, preferably asdescribed in U.S. patent application Ser. No. 09/365,963, which isassigned to the assignee of the present patent application and whosedisclosure is incorporated herein by reference. BSS 32 preferably alsocomprises a radio operation and maintenance center (OMC-R) 38, whichcommunicates with NMC 26.

Communications between CDMA BSS 32 and MS 40 are built on a CDMA “airinterface,” which is preferably generally in accordance with the IS-95standard for CDMA communications. BSS 32 is built around a base stationcontroller (BSC) 34, which controls and communicates with a number ofbase station transceivers (BTS) 36. Each BTS transmits RF signals to andreceives RF signals from MS 40 when the MS is within a geographicalarea, or cell, served by the particular BTS. When entering idle mode,either immediately after switch-on or in between calls, the MS selectsand camps on one of the cells. While in the idle mode, the MScontinually monitors other, generally neighboring, cells to determinewhether to select a new cell on which to camp.

FIG. 2 is a schematic map of overlapping GSM/TDMA cells 47 and GSM/CDMAcells 49 in system 20, illustrating aspects of idle-mode cell selection,in accordance with a preferred embodiment of the present invention. WhenMS 40 is in a region served only by TDMA cells 47 (i.e., associated withGSM BSS 30), it will select and camp on one of these cells and willperiodically monitor signals received from neighboring cells todetermine whether cell reselection (selection of a new TDMA cell to campon) is called for. When the MS moves into any of cells 1-5 shown in FIG.2, however, it may also monitor CDMA signals from cells associated withCDMA BSS 32. If appropriate, the MS will select one of the CDMA cells tocamp on, so that not only the cell, but also the air interface, isreselected. Cells 3, 4 and 5 would be considered “border cells,” inwhich MS 40 is known to be reaching the end of the region served byGSM/TDMA BSSs and in which reselection of the air interface may berequired. Similar processes of cell and air interface reselection takeplace when the MS is served by one of the CDMA cells.

Methods for performing such monitoring, cell selection and reselectionin system 20 are described further hereinbelow. Methods for performinghandovers between GSM/CDMA and GSM/TDMA service and vice versa indedicated mode (during a call), as well as between one CDMA BSS 32 andanother in system 20, are described further in the above-mentioned U.S.patent application Ser. No. 09/365,967. By virtue of such methods and ofthe architecture of system 20, as shown in FIG. 1, MS 40 receives thebenefits of CDMA service in those regions served by system 20 in whichthe service has been implemented, without losing service in TDMAregions. Transitions between CDMA and TDMA regions are substantiallytransparent to users of MS 40, because higher-level GSM networkprotocols are observed throughout the system, and only the lower-levelRF physical interface is changed during the transition.

Mobile Station Structure and Protocols

FIG. 3 is a block diagram that schematically illustrates communicationsprotocol stacks between MS 40 and BSSs 30 and 32, in accordance with apreferred embodiment of the present invention. MS 40 communicates withGSM BSS 30 over a GSM/TDMA air interface, in accordance with GSMstandards substantially without modification. Therefore, substantiallyno modification is required to BSS 30 or to GSM Layer 1 and Layer 2standard interface protocols, indicated by blocks 53 and 54 in thefigure, in order to accommodate MS 40. MS 40 communicates with CDMA BSS32 over a CDMA air interface, preferably based on a CDMA IS-95 airinterface with certain modifications. Mobile stations known in the artare capable of operating over either a GSM air interface or a CDMA airinterface, but not both.

In order to sustain both of these interfaces, MS 40 comprises mobileequipment (ME) 42 (FIG. 1), which includes either two radiotransceivers, one configured for TDMA operation and one for CDMA, or asingle transceiver which can dynamically switch between TDMA and CDMA.The ME also includes mobile termination (MT), which supports terminalequipment (TE) 46 for voice and/or data input and output. In addition,MS 40 comprises a subscriber identity module (SIM) 44, in accordancewith GSM standards.

FIG. 4 is a schematic block diagram illustrating MS 40, which herecomprises a single radio transceiver in ME 42, in accordance with apreferred embodiment of the present invention. MS 40 is built around amodem unit 59, including a DSP core 60 capable of generating andprocessing both TDMA and CDMA signals. Preferably, core 60 comprises anASIC device, including stand-alone CDMA transmission/receptionprocessing, which is supported by GSM timing logic 64 and a GSM hardwareaccelerator (or DSP) 62, as well as having a port for SIM 44. Core 60receives input and delivers output to TE 46. In this case, TE 46 isrepresented as an audio microphone and speaker, and core 60 performs D/Aand A/D conversion, as well as vocoding functions on the audio signals,as are known in the art. Core 60 may, additionally or alternatively, beconfigured to work with TE 46 providing digital data input/output, suchas a fax device.

Core 60 outputs digital data, which may be in either TDMA or CDMAformat, to a mixed-signal output device 66. Device 66 processes andconverts the data to analog baseband form, for input to RF transmitter68. A duplexer 70 conveys the resultant RF signals via antenna to theGSM or CDMA base station, as appropriate. Signals received from the basestation are passed by duplexer 70 through an RF receiver 72 and amixed-signal input device 74, which performs baseband conversion and AGCfunctions, to core 60. Preferably, transmitter 68, receiver 72 andmixed-signal devices 66 and 74 are controlled by core 60.

In a preferred embodiment of the present invention, SIM 44 of GSM/CDMAMS 40 comprises an extended read/write memory. This memory is used torecord date and programs used in GSM/CDMA operation, which are notrequired for or supported by conventional GSM operation. Additionally oralternatively, the MS includes an integral non-volatile memory,independent of the SIM, in which such data and programs are stored.

RF transmission and reception by MS 40 are preferably at frequencies inthe GSM 900 and/or 1800 MHz bands, for compatibility with existing GSMequipment, particularly BSS 30. Preferably, transmitter 68 and receiver72 are dual-band devices, capable of operating in both of the GSM bands.Assuming that MS 40 includes only the single transceiver shown in FIG.4, operating in the GSM band, CDMA equipment in system 20 must beappropriately configured to operate in these frequency ranges, as well.

While MS 40 is in idle mode, i.e., not engaged in setting up or carryingout a call, it camps on and actively “listens” to messages from a cellbelonging to either GSM BSS 30 or CDMA BSS 32, so as to be prepared toreceive a paging message from the appropriate BSS. The cell on which theMS is camped is referred to herein as the serving cell. Methods by whichthe MS selects one or the other of the air interfaces and selects thecell on which to camp are described further hereinbelow. The MS alsomonitors other cells belonging to the selected air interface, referredto herein as the active interface, in order to determine whether toselect a different serving cell. In order to reduce power consumption inidle mode, typically only one of the air interfaces is active at anygiven time. The other air interface is referred to herein as the passiveinterface. Under certain circumstances, which are also describedhereinbelow, the MS monitors signals from cells on the passive airinterface, whereupon the passive interface may be reselected to be theactive interface, and vice versa.

Returning to FIG. 3, whether MS 40 physically includes one transceiveror two, it must support dual air interface Layers 1 and 2 in itsprotocol stack, for operation vis-a-vis GSM BSS 30 and CDMA BSS 32,respectively. As noted above, at any given time, one of these airinterfaces is selected to be active, and the other is passive. The CDMAair interface between MS 40 and CDMA BSS 32 comprises CDMA Layer 1(block 51 in the figure), also known as the physical layer, whichpreferably operates on a standard IS-95 protocol, and CDMA data linkLayer 2 (block 52), preferably based on IS-95, with appropriatemodification to accommodate the needs of GSM network services.

In idle mode operation, the GSM/CDMA physical layer in the MS 40monitors broadcast channels of CDMA BTS 36 for messages addressed to theMS, operating in either slotted or non-slotted mode as appropriate, asdescribed in the above-mentioned U.S. patent application Ser. No.09/365,963. This occurs as long as the CDMA air interface is active, orwhen the GSM air interface is active but conditions have arisen in whichcells are to be monitored on the passive CDMA air interface, as well.The physical layer passes messages to the upper protocol layers incoordination with the slotted mode operation of the MS.

When the CDMA air interface is active, CDMA Layer 1 measures thestrength of a pilot beam associated with the serving cell and apredetermined number of neighbor cells belonging to the activeinterface, preferably six such cells. The measurement is preferablyrefreshed about once per second. On the other hand, when the GSM/TDMAair interface is the active one, CDMA Layer 1 can also be called upon bya GSM/CDMA RR sublayer 55 (described further hereinbelow) to monitor thetiming, quality, RF signal strength and cell information of neighboringcells belonging to the passive CDMA interface. These functions aresubstantially similar to those carried out by GSM/TDMA Layer 1 inaccordance with GSM specifications.

CDMA Layer 2 preferably conforms to IS-95 specifications, but alsoincludes functionality, such as message ordering, priority andfragmentation, and suspension and resumption of communications, which isnormally supported by the standard GSM Layer 2, but not by CDMA IS-95.In particular, while MS 40 is in idle mode, CDMA Layer 2 supports bothunacknowledged operation on paging and access channels, in accordancewith IS-95 specifications, and acknowledged operation for idle modechannel request, which is a feature of GSM specifications. The designand operation of such a CDMA Layer 2 are described in greater detail ina U.S. patent application entitled, “Signaling Data Link for a GSM-CDMAAir Interface,” filed Sep. 29, 1999, which is assigned to the assigneeof the present patent application and whose disclosure is incorporatedherein by reference. Vis-a-vis GSM BSS 30, air interface Layers 1 and 2are in accordance with GSM standards, substantially withoutmodification.

As noted in the Background of the Invention, standard GSM protocolsinclude a Radio Interface Layer Three (RIL3), including threesub-layers, above GSM Layer 1 and Layer 2. The lowest of these threeRIL3 sub-layers is a Radio Resource (RR) management sub-layer, whichprovides services to Mobile Management (MM) and Connection Management(CM) sub-layer above it. The RIL3 sub-layers in GSM BSS 30 aresubstantially unchanged with respect to the GSM standard, and the GSM MMand CM sub-layers are likewise maintained substantially without changein MS 40. The CM sub-layer supports signaling for call processing, aswell as GSM supplementary services and short message service (SMS). TheMM sub-layer supports signaling required for updating the location of MS40 and PLMN selection, as described further hereinbelow, andcommunicates with SIM 44.

In order to support the unmodified upper MM and CM sub-layers, GSM-CDMARR sub-layer 55 is introduced in the MS 40 and BSS 32 protocol stacks.The GSM/CDMA RR sub-layer in the MS, which manages radio resources andmaintains radio links between MS 40 and BSSs 30 and 32, is “aware” ofthe existence of the dual GSM and CDMA lower layers (Layers 1 and 2) inthe MS 40 protocol stack. It invokes the appropriate lower layers in theMS stack to communicate with either the standard RIL3-RR sub-layer ofBSS 30 over the GSM air interface or the GSM/CDMA RR sub-layer of BSS 32over the CDMA air interface. The MM and CM sub-layers are not processedby BSS 32, but are rather relayed through between MS 40 and MSC 24 forprocessing in a manner substantially transparent to the CDMA airinterface layers below.

GSM/CDMA RR sub-layer 55 supports standard GSM RIL3-MM and CM sub-layers56 and 57 above it regardless of which of the air interfaces is in use.The RR sub-layer preferably offers complete radio resource managementfunctionality as defined by GSM specifications 04.07 and 04.08, whichare incorporated herein by reference. Although a “RR” sub-layer per seis not defined by the CDMA IS-95 standard, the GSM/CDMA RR sub-layerdescribed herein preferably maintains full IS-95 radio resourcefunctionality, as well.

In accordance with GSM standards, the functionality of the RR sub-layerincludes both idle mode operation and dedicated mode services (i.e.,services performed during a telephone conversation). The idle modeoperation of the RR sub-layer includes automatic cell selection andreselection (idle handover) between GSM and CDMA cells, as well asbetween pairs of CDMA cells and pairs of GSM cells, with cell changeindication as specified by GSM standards, and particularly GSM standard05.08. The RR sub-layer in idle mode also performs monitoring ofneighboring cells, broadcast channel processing, as specified by GSM andCDMA standards, and establishment of RR connections.

It will be understood by those skilled in the art that the abovefeatures of RR sub-layer 55 are listed only by way of a summary, andthat additional details and features may be added based on published GSMand CDMA specifications.

FIG. 5 is a block diagram illustrating details of GSM/CDMA RR sub-layer55, in accordance with a preferred embodiment of the present invention.The RR sub-layer preferably includes separate GSM and CDMA idle modeprocesses, labeled RRG 80 and RRC 84, respectively, in the figure. TheRRG and RRC processes are together referred to hereinbelow as the RRXprocesses. Each of the processes is responsible for idle modecommunications of MS 40 with BTSs of the respective type and interfaceswith the respective GSM/TDMA or GSM/CDMA Layer 2. Operation of the RRGand RRC processes is coordinated by a RR combiner (RRCO) process 84.

The RR combiner interfaces to the MM sub-layer above it through asubstantially standard GSM service access point (SAP), preferably asdefined by GSM standard 04.07. Thus, MM sublayer 56 can be programmedand can operate entirely in accordance with GSM standards, substantiallywithout modification. Services provided by the RR sublayer to the MMsublayer, as required by GSM standards, preferably include:

-   -   Indicating to the MM sublayer when the paging channel of the        serving cell is unavailable.    -   Accepting service requests from the MM sublayer and indicating        to the MM sublayer if a failure occurs in accessing the cell.    -   Reporting to the MM sublayer any changes of relevant cell        information.    -   Reporting to the MM sublayer upon successful cell selection, as        well as upon failing to select a cell because there is no        service available.    -   Generating a list of available PLMNs when requested by the MM        sublayer.

Preferably, both the RRG and RRC processes respectively includesubstantially the full functionality of the GSM RR sub-layer or theequivalent IS-95 radio resource management capabilities. In idle mode,this functionality includes cell selection and reselection, as describedfurther hereinbelow. The RRCO process (in idle mode) is then largelylimited to selection and reselection of the active air interface:GSM/TDMA or GSM/CDMA. This approach is advantageous in that it can beimplemented easily and quickly, using existing RRG and RRC programcodes. Alternatively, more efficient use of memory may be obtained byreducing the functionality of the RRG and RRC processes and programmingthe RRCO process to perform cell selection and reselection.

In accordance with the features of idle mode operation of MS 40described hereinabove, one of the RRG and RRC processes is defined asthe active process, depending on whether the MS is camped on a TDMA or aCDMA cell. The active process controls the transceiver in ME 42 (FIG. 4)and executes appropriate camped cell activity, in accordance with GSMstandards. The other one of the processes is the passive process, and ispreferably limited to the minimal activity needed to support periodicmonitoring on the passive air interface and possible cell/air interfacereselection, in order to reduce power consumption by the MS and thusprolong battery life. If extended cell broadcast information regardingthe second air interface is transmitted by the serving cell, the activeprocess also receives and passes the relevant information to the RRCO.

In order to decide whether to perform an air interface reselection, theRRCO intermittently passes a measurement request to the passive process,which then makes the measurement and passes the results back to theRRCO. Preferably, the measurement is made during “sleep periods” of theactive process operating in DRX (discontinuous receive) mode, asreported to the RRCO by the active process. The RRCO compares themeasurements received from the passive mode process with measurementsmade and passed to the RRCO by the active mode process and based on thecomparison, decides when to make an air interface reselection.

FIG. 6 is a block diagram that schematically illustrates message flowbetween RRCO process 84 and other layers and sublayers in MS 40 and inBSSs 30 and 32. As noted hereinabove, RRCO 84 communicates with MM layer56, preferably via a RR SAP 90, which is substantially in accordancewith GSM protocol standards, particularly GSM standard 04.07. RRCO 84maps MM requests conveyed via SAP 90 into state variables appropriate towhichever of processes RRG 80 and RRC 82 is active and then downloadsthe state variables to the RRG or RRC process. Communications over SAP90 are based on service primitives defined by the GSM standard, and mayinclude certain additions for enhanced GSM/CDMA operation. Theprimitives and associated parameters conveyed between RRCO 84 and MMsublayer 56, as well as between RRCO 84 and RRG 80 or RRC 82 aredescribed in Appendix C.

GSM standard 03.22 defines idle mode behavior of the MS, andparticularly of the RR protocol sublayer of the MS, in terms of a dualstate machine, having normal and limited service modes. For each suchservice mode, the MS begins in a select state (which may be a “normalselect,” “stored list select,” or “choose select” state, as defined inthe Background of the Invention), from which the MS attempts to selectand camp on a suitable cell, in a camp state. When necessary, the MSenters a reselect state, wherein a new cell is selected to camp on.

In a preferred embodiment of the present invention, each of the GSMstates of the RR sublayer is mapped to a corresponding state of theactive RRX process (i.e., RRG or RRC) and of the RRCO process. States ofthe RRX and RRCO processes and transitions among the states are furtherdescribed hereinbelow with reference to FIGS. 8 and 9.

Overview of Idle Mode Procedures

Reference is now made to FIG. 7, which is a flow chart thatschematically illustrates idle mode behavior of GSM/CDMA MS 40, inaccordance with a preferred embodiment of the present invention.

Upon initialization (switch-on), MS 40 enters a PLMN selection state100, in which it selects a PLMN, under the control of MM protocolsublayer 56, typically in accordance with PLMN selection criteriadefined by GSM standards. As noted above, if the MS is unable to selecta PLMN (as required for normal service), it will enter the limitedservice mode, according to the GSM limited service criteria. MS 40 thenbegins idle mode monitoring of signals received from cells in itsgeographical area, over either one or both of the GSM/TDMA and CDMA airinterfaces. The MS enters an interface selection state 102, in which itselects one of the air interfaces to be the active one, based on themonitoring results and interface selection criteria describedhereinbelow.

Having selected the air interface, and having successfully selected aPLMN, MS 40 enters a normal cell selection state 106 or a stored listcell selection state 107, in which it attempts to select a cell thatfulfills suitable cell criteria for the selected active air interface.Cell selection may also involve band selection, such as between the GSM900 and 1800 MHz bands. The cell selection states, along with a campingstate 108 and a cell reselection state 110, belongs to a group of statesthat are specific to the selected air interface. In other words, whenthe GSM/TDMA air interface is selected, the behavior of the MS in thesestates and the decision criteria for making transitions among the statesare substantially in accordance with relevant GSM standards. On theother hand, when the GSM/CDMA air interface is selected, the behaviorand decision criteria, as described hereinbelow, may differ, althoughthey are still similar to the GSM standards.

Whichever air interface has been selected, the MS continues to searchfor a suitable cell, but gives up if it has not found a cell and “nosuitable cell” criteria are met for the active interface. (If the MS isin stored list selection state 107 and reaches the end of the storedlist of available cells without finding a suitable cell, it firstproceeds to normal selection state 106 and continues the search.) Inthis case, the MS returns to interface selection state 102, chooses theother (passive) air interface to be the active one, and then returns tocell selection state 106 or 107. If the MS gives up on cell selectionover both air interfaces, a “no service” indication is passed back tothe upper protocol layer (MM).

Upon successful cell selection (again, assuming a PLMN to have beensuccessfully selected), the MS enters state 108, in which it camps onthe selected cell and carries out normal camped cell activity asappropriate. When necessary upon cell selection or reselection, alocation update is performed, in accordance with GSM standards.Periodically, the MS monitors neighboring cells on the active airinterface, based on monitoring criteria described hereinbelow. When cellreselection criteria are met, the MS enters state 110, in which itattempts to perform cell and/or band reselection based on cellreselection criteria appropriate to the active interface. If a cell isfound (either the current cell or a new cell on the same air interface)that meets the suitable cell criteria, the MS returns to state 108 andcamps on the current or new cell. If the “no suitable cell” criteria aremet, however, the MS re-enters state 106, and may from there return tointerface selection state 102 if necessary.

While the MS is in state 108, it may also begin monitoring of cells onthe passive air interface when predetermined passive interfacemonitoring criteria are met, as described further hereinbelow. Based onmeasurements made on the passive interface, the MS assesses thenecessity for interface reselection based on interface reselectioncriteria. If the criteria are met, the MS enters an interfacereselection state 112. If a cell is found on the passive air interfacethat is preferable to the current, active cell, the MS enters a newinterface cell selection state 113. If the new cell is successfullyselected in this state, the MS goes directly to camping state 108.Alternatively, the MS returns to cell selection state 106 to select anew cell belonging to the new active interface. On the other hand, ifthe passive interface monitoring criteria are met, but the MS determinesthat there is no need for a interface reselection, the MS subsequentlyre-activates passive interface monitoring periodically, preferably usinga hysteresis timer to prevent constant, rapid cycling, which runs downthe MS battery.

As noted hereinabove, while MS 40 is idle in camping state 108, it isprepared to enter dedicated mode service in a connected state 115, afterreceiving an appropriate paging message or a service request from theupper MM and CM layers. Dedicated mode behavior of the MS is describedin the U.S. patent applications described hereinabove and is beyond thescope of the present patent application. Upon termination of dedicatedservice, the MS returns to idle mode, preferably via a choose cellselection state 105 and, if necessary, via normal cell selection 106,followed by interface selection 102.

The description up to this point has related to interface and cellselection under conditions in which PLMN selection was successful. Ifthis is not the case, the MS enters limited service mode, as describedin the Background of the Invention, in which it attempts to select andcamp on any PLMN, air interface and cell that will allow it to makeemergency calls. Cell selection in this case is carried out via “anycell” selection state 117 or a “choose any” state 116 following a callin an emergency connect state 114. If the cell selection is successful,the MS camps on the selected cell in a camping state 109. Behavior ofthe MS is this state, in terms of cell and interface reselection anddedicated mode service when required, is largely similar to thatdescribed hereinabove with regard to normal service, subject toconstraints imposed by the limited service. Likewise, states 114, 116,118 and 119 associated with the limited service mode are similar to thecorresponding normal service states 115, 105, 110 and 113, respectively.If the MS succeeds in selecting a PLMN, it returns to the appropriate,normal cell selection and camping states.

FIG. 8 is a flow chart illustrating behavior of the RRX processes (RRGprocess 80 and RRC process 82, as shown in FIG. 5 and described withreference thereto) during the idle mode operation of MS 40 illustratedin FIG. 7. To the extent appropriate, the states of the RRX process inFIG. 8 are identified by the same names and indicator numbers as thecorresponding states of the MS in FIG. 7. Where appropriate in FIG. 8,state transitions are labeled with service primitives associatedtherewith, as listed in Appendix C.

Each of the RRX processes begins in an idle passive state 120, eitherwhen MS 40 is turned on or following selection of the RRX process to bethe active one. While in the passive state, the RRX process periodicallymonitors cells over its corresponding air interface, when instructed bythe RRCO process to do so. In response to a RRX_ACT_REQ message from theRRCO process, the RRX process becomes active, and the MS enters theappropriate cell selection state 106, 107 or 117. From this point on, aslong as the RRX process is active, its behavior and state transitionsessentially mirror the MS idle mode behavior and states shown in FIG. 7.When it is determined, however, that the other, passive air interfaceshould become the active one, the RRCO process passes aRRX_GO_IDLE_PASSIVE_REQ message to the RRX, which then returns to idlepassive state 120, regardless of the state that the RRX was inbeforehand.

FIG. 9 is a flow chart illustrating behavior of the RRCO process duringthe idle mode operation of MS 40 illustrated in FIG. 7. Here, too, tothe extent appropriate, the states of the RRCO process in FIG. 9 areidentified by the same names and indicator numbers as the correspondingstates of the MS in FIG. 7. Wherever appropriate in FIG. 9, statetransitions are labeled with service primitives associated therewith, aslisted in Appendix C, including both the RR_SAP primitives and theRRCO-RRX primitives.

As noted earlier, when the MS is switched on, RRCO begins in a state 122in which both of the RRX processes are passive. When the RRCO processreceives a RR_ACT_REQ primitive from an upper protocol layer, it entersinterface selection state 102. Once the air interface is selected, theactive RRX process then goes on to select a cell, while the RRCO processwaits in an appropriate “one selecting” state 124 or 126 (depending onwhether a PLMN was selected, or alternatively, whether the MS isoperating in limited service mode, as described hereinabove). When acell is selected, the RRCO process enters a “one camped” state,corresponding to camped states 108 or 109 of the MS.

In these camped states, RRCO intermittently invokes passive airinterface monitoring by the passive RRX process, based on monitoringcriteria described hereinbelow. When the interface reselection criteriaare met, the RRCO process passes to interface reselection state 112, andfrom there back to selecting state 124 or 126. At this point the passiveand active RRX processes have switched, but the states of RRCO are thesame regardless.

Procedures for GSM/CDMA Cell Selection and Camping

Certain of the states and processes illustrated in FIGS. 7-9, associatedwith the cell selection/reselection and camping processes, will now bedescribed in greater detail. Aspects of the operation of MS 40 that areentirely in accordance with GSM specifications or that can be derivedfrom GSM specifications in a straightforward manner are omitted.

FIGS. 10A and 10B are flow charts that schematically illustrateprocedures associated with cell selection states 105, 106 and 107, whenthe CDMA air interface has been selected, in accordance with a preferredembodiment of the present invention. The procedures generally apply, aswell, in the corresponding “any cell” selection states in the limitedservice mode. Similar procedures are followed for cell selection overthe GSM/TDMA air interface, in accordance with GSM standards, butdifferent parameters and decision criteria are involved.

MS 40 scans the spectrum of CDMA frequencies in all supported frequencybands, in order to detect strong pilot signals and to build a list ofthe pilots in order of measured RF power. The list of cells whosefrequency the MS scans can include neighbor cells in the last servingPLMN (stored list cell selection state 107), neighbor cells monitoredduring the last connection (choose cell selection state 105) or allsupported CDMA Frequency Assignments (CFAs) in all supported bands ofoperation (normal cell selection state 106). Preferably, the MSmaintains a list of “forbidden location areas (LAs) for roaming,” whichis updated whenever the MS is informed that a particular LA isforbidden, in accordance with GSM standards. The MS does not attempt toreceive service in cells belonging to the forbidden LAs.

The MS then searches the cells on the list in order. If the cellfulfills the “suitable cell criteria” defined hereinbelow, the MS campson that cell. Otherwise, it attempts to select the next cell on thelist.

If the MS reaches the end of the list, or when the “no suitable cellcriteria” are fulfilled in stored list or choose cell selection, itattempts normal cell selection, or “any cell” selection (state 117) ifin limited service mode. If this, too, is unsuccessful, the cellselection process returns to interface selection state 102.

Preferably, in accordance with IS-95 standards, after finding a suitablepilot channel of a CDMA cell, the MS then attempts to receive a validsync channel message from the cell. The MS synchronizes its long codeand system timing to those of the CDMA cell, using PILOT_LPN, LC_STATE,and SYS_TIME values derived from the received sync channel message. Itthen reads the full set of system overhead messages on the pagingchannel of the cell. If the MS receives a page before having decoded thefull set of messages, it preferably stores the page and responds afterall of the messages have been decoded, provided that the response is notbarred for any reason.

Optionally, MS 40 performs band selection in conjunction with cellselection, generally as provided by GSM standards. Candidate cellssearched in stored list cell selection, as described above, may belongto one band or to multiple frequency bands. In normal cell selection, onthe other hand, the MS searches all channels in all supported bands ofoperation in order of their respective RF signal strengths, using apredefined order of band preference. For purposes of CDMA bandselection, MS 40 preferably refers to one or more preference lists:

-   -   Last active band and CDMA frequency assignment (CFA, maintained        in the extended memory on SIM 44 by the MS).    -   A list of preferred CDMA bands and CFAs (stored in extended        memory on SIM 44 of the MS).    -   A list of supported CDMA bands (pre-configured in the MS).

In stored list cell selection, associated with state 107 (FIG. 7), MS 40refers to a predefined neighbor list, for example, a list of carrierfrequencies defined by their CDMABAND and CDMACH parameters, inaccordance with the IS-95 standard, for the selected PLMN. Preferably,the neighbor list is one that was provided to the MS by the last-usedPLMN and stored in the memory on the extended SIM, as describedhereinabove, or in a non-volatile memory of the MS. If there is a validLocation Area Identifier (LAI) stored in the SIM, then the neighbor listmust belong to the PLMN indicated by the LAI. In the course of searchingand attempting to camp on the cells in the list, if the MS is able todecode the messages of a cell of the selected PLMN but cannot camp onthe cell, the neighbors of that cell are preferably added to the list.

When selecting a cell in “choose cell” selection state 105, the MSpreferably attempts to camp on the last serving cell from the periodthat the MS was operating in dedicated mode. If the last serving cell isknown to be unsuitable (i.e., it fails to meet the “suitable cellcriteria” described hereinbelow), then the MS can attempt to camp on anyof the surrounding cells. For the purpose of ordering the list ofsurrounding cells, the MS preferably measures the power on the trafficchannels of each of the cells while in dedicated mode, averaging themeasurements over a predetermined period, typically 5 sec. If the fullset of overhead messages for the selected cell was already decodedshortly before selection, for example, within the preceding 30 sec, thenit is generally not necessary for the MS to decode them again, contraryto what is indicated in FIG. 10B. Furthermore, when the MS is requestedto re-establish an interrupted call, the time required for cellselection is preferably shortened by omitting non-essential steps in thecell selection method.

In new interface cell selection state 113, cell selection is preferablyperformed in a manner generally similar to that in choose cell state105. The similarity is made possible if in the course of monitoring thepassive air interface before the interface reselection, the MS madepower measurements of neighbor cells. These measurements are used toassemble the list from which the new serving cell is selected.

Cell selection in limited service, carried out in “any cell” selectionstate 117, “choose any” state 116, and “new interface any” selectionstate 119, is largely similar to the parallel, respective normal cellselection state 106, choose cell state 105 and new interface normalselection state 110, with changes as required for limited serviceoperation.

FIG. 11 is a flow chart that schematically illustrates cell reselectionprocedures followed by MS 40, in accordance with a preferred embodimentof the present invention. While camped on a cell, MS 40 periodicallymeasures signal strengths or qualities of neighboring cells. Theneighboring cells are preferably searched in accordance with a list ofneighbor channels that is broadcast by the serving cell. The MS attemptsto acquire the cell with the best available pilot. The MS thendetermines whether reselection is necessary based on “cell reselectioncriteria,” as described hereinbelow.

When the criteria are met, the MS triggers a cell reselection. Beforecamping on the new cell, the MS preferably decodes the full set ofoverhead messages and evaluates the cell parameters. If the new cell islisted in a neighbor list parameters message broadcast by the old cell,the MS can preferably make a pilot-to-pilot transition, skippingdecoding of the sync channel, as defined by the IS-95 specification.Otherwise, the sync channel message of the new cell is preferablydecoded first. Once it has acquired the new cell, the MS preferablyoperates in a non-slotted mode, as provided by the IS-95 specification,until it has received at least one valid message on the new pagingchannel. The MS then camps normally on the new cell.

If after beginning reselection, the MS fails to find a suitable cellwithin a predetermined period, preferably about 10 sec, it returns tonormal cell selection state 106.

It is noted that the description above applies generally to both normalcell reselection, associated with state 110, and “any cell” reselection,associated with state 118 in limited service mode, with appropriatechanges to the reselection criteria.

Cell selection and reselection over the CDMA air interface arepreferably based on a CDMA path-loss criterion (C1 c) and a reselectioncriterion (C2 c). The path-loss criterion is used to determine whetherthe MS can communicate with the network without interference, i.e.,whether the MS is located within an area of good coverage by the cell inquestion. The reselection criterion is used to determine a relativequality level of candidate cells so as to find the best cell available.It uses C1 c and additionally takes into account a cell priorityallocated by the network (CELL_RESELECT_OFFSET).

C1 c is given by the total received power spectral density at theantenna connector of MS 40, measured for the pilot of a particular cell:

C1 c=−20 log₁₀(E_(C)/I_(O)),

Wherein it is preferable that:C1c>EC _(—) IO_THRESHThe terms E_(C), I_(O) and EC_IO_THRESH are taken from the IS-95standard, wherein E_(C)/I_(O) is the ratio in dB between the pilotenergy accumulated over one PN chip period (E_(C)) to the total powerspectral density (I_(O)) in the received bandwidth.

Furthermore, for a cell to be selected, the pilot power preferablysatisfies:Pilot_power>EC_THRESH−115,wherein PiloLpower (in dBm/1.23 MHz) is defined as follows:Pilot_power=−20 log₁₀(E _(C) /I _(O))(dB)+mean input power (dBm/1.23MHz)Preferably, the values of E_(C)/IO_THRESH and EC_THRESH are broadcast toMS 40 as part of a CDMA extended system parameters message.

The reselection criterion C2 c for CDMA is defined as:C2c=C1c−CELL_RESELECT_OFFSETCELL_RESELECT_OFFSET is a cell reselection parameter similar to one usedin GSM, which is preferably broadcast in the cell, as provided by GSMstandard 05.08.

When a neighboring cell has a higher value of C2 c than the currentserving cell for a predetermined time period, typically 5 sec, cellreselection is invoked, so long as the neighboring cell is in the samelocation area as the serving cell. For a neighbor belonging to the sameair interface but having a different location area, the condition ispreferably:C2c(new cell)>C2c(current cell)+CRHwherein CRH is a cell reselection hysteresis factor, added to preventtoo-frequent location area changes, which consume substantial batterypower in the MS. Use of this criterion requires that the MS be awarethat the neighboring cell belongs to a different location area. Inpreferred embodiments of the present invention, there are twoalternative ways by which the MS can gain awareness of the location areaof the neighboring cell:

-   -   The neighbor list broadcast by the current serving cell can        include location area information.    -   The MS can store in memory the location areas of cells that have        served it in the past, for later reference in cell reselection.

Similarly, if there has been a recent cell reselection, additionalconstraints are preferably placed on the reselection criteria. Forexample, if a reselection took place within the past 15 sec, C2 c of thenew cell is required to exceed C2 c of the current cell by at least 5 dBfor 5 sec, and the MS should not return to the same cell within 4 sec ifanother suitable cell can be found. In any event, the neighboring cellthat is to be selected must also satisfy the path-loss requirementsimposed on C1 c and on the pilot power.

While operating over the CDMA interface, MS 40 will preferably selectonly cells meeting the CDMA suitable cell criteria. The criteria arebased generally on suitable cell criteria defined by the GSM standard. Acell is suitable for the MS to camp on if it fulfills all of thefollowing conditions:

-   -   The cell satisfies the requirement described above regarding the        C1 c path-loss criteria; and    -   The MS is able to detect the pilot channel signal of the cell        within a predetermined period, preferably about 15 sec; and    -   The MS receives a valid message on the cell's sync channel        within a predetermined period, preferably about 1 sec; and    -   The MS is able to read the full set of overhead messages of the        cell within a predetermined period, preferably about 4 sec; and    -   The cell belongs to the selected PLMN (or, when in limited        service mode, the cell belongs to any PLMN and supports        emergency calls); and    -   The cell is not barred (i.e., the MS is permitted to access the        cell); and    -   The cell is not in the “forbidden LAs” list, as described        hereinabove; and    -   The cell has a normal priority allocated, as defined by GSM        standards, unless a predetermined number of channels have        already been searched, and all suitable cells have low priority        as determined by a CELL_BAR_QUALIFY parameter broadcast by the        network. Preferably, five channels per CFA are searched before        low-priority cells are admitted.

Cell reselection is also invoked if the serving cell itself does notfulfill the above criteria; or if the paging channel of the serving cellis lost for a predetermined period of time, typically several seconds;or if a service attempt on the access channel of the serving cell hasfailed.

The “no suitable cell” criterion for the CDMA air interface is likewisebased on the criterion used in GSM: In normal cell selection state 106and “any cell” selection state 117, the MS gives up searching for cellsbelonging to the current air interface after having searched apredetermined number of RF channels, and finding none that fulfill thesuitable cell criteria. In the other cell selection states shown in FIG.7, which use predetermined lists of cells, when the MS reaches the endof the list of cells associated with the state, as describedhereinabove, it enters state 106 or 117, as appropriate, in order tocontinue the search.

While in normal camping state 108, MS 40 preferably performs thefollowing activities:

-   1. The MS monitors the paging channel of the serving cell and    indicates to the upper MM protocol layer of the MS if the paging    channel becomes unavailable.-   2. The MS decodes the broadcast system overhead messages of the    serving cell and indicates to the upper layer any changes in the    broadcast of relevant parameters.-   3. While camped on a cell and having a valid SIM 44 inserted, the MS    listens to all paging messages that could be addressed to it, as    required by GSM standard 05.02.-   4. The MS listens for cell broadcast messages subscribed to by the    user, in accordance with the principles of GSM short message service    (SMSCB), particularly as described in the above-mentioned U.S.    patent application Ser. No. 09/365,963.-   5. The RR layer in the MS accepts service requests from the upper    layer of the MS and indicates to the upper layer if there is a    failure to access the cell.-   6. The MS periodically assesses the cell reselection criterion, as    described above, and initiates cell reselection if necessary.-   7. Furthermore, the MS also initiates cell reselection if one of the    other relevant criteria described above is realized, for example, if    the current serving cell becomes barred, or if there is a downlink    signaling failure.-   8. In national roaming, in accordance with GSM standards, the MS    supports periodical home PLMN (HPLMN) search.-   9. The MS generates a list of available PLMNs when so requested by    the upper layer, preferably generating the list in a way that    minimizes interruptions in monitoring of the paging channel by the    MS.-   10. The MS preferably supports dual-interface monitoring and air    interface reselection, as described hereinbelow. When appropriate,    based on passive interface monitoring criteria, the MS initiates    signal strength measurements over the passive air interface (passive    interface monitoring) in order to determine whether interface    reselection is called for.-   11. Once passive interface monitoring has been initiated, the MS    periodically assesses appropriate interface reselection criteria and    initiates interface reselection when appropriate.-   12. Preferably, one of the air interfaces is designated in the    memory of the MS (in the extended SIM or in integral, non-volatile    memory) as the preferred interface. In this case, when the MS camps    on a cell belonging to the non-preferred interface, it preferably    runs an interface search timer and performs a periodic interface    search even when other interface reselection criteria are not met.

In “any cell” camped state 109, as noted hereinabove, the MS camps onany cell from which emergency calls can be made if necessary. In thisstate, the MS performs cell reselection similar to that associated withnormal camped state 108, except that the hysteresis parameter CRH ispreferably set to zero. If the MS has a valid SIM (despite being in thelimited service mode), it intermittently searches for available andallowable PLMNs, as described generally in GSM standards 03.22 and02.11.

Air Interface Selection and Reselection

FIG. 12 is a flow chart that schematically illustrates air interfaceselection procedures followed by the MS, in accordance with a preferredembodiment of the present invention. Air interface selection is invokedwhenever a new PLMN is selected (by the MM sub-layer) or when nosuitable cell is found on the active air interface during normal cellselection.

The MS initially selects one of the air interfaces to be the activeinterface, and attempts to camp on a cell transmitting over thatinterface. Preferably, the MS selects the last active interface, whenknown. Otherwise, the MS selects the user's preferred interface asprogrammed on the SIM, or a default preferred interface as programmed onthe non-volatile memory of the MS. Optionally, if the MS is located inan area in which it has no prior information regarding coverage by oneair interface or another, the order of mode selection is preceded by aninitial power measurement over both interfaces.

If unsuccessful in camping on a cell over the first air interface, theMS switches to the other air interface and seeks a suitable cell forcamping there. Successful selection and camping are reported to upper(MM) protocol sublayer 56. Failure to find a cell to camp on over bothinterfaces is likewise reported to the MM sublayer, and the entireprocess is re-attempted at a later time.

After MS 40 has successfully selected an air interface and camped on acell, air interface reselection is invoked when interface reselectioncriteria are met, as described hereinbelow. The criteria are assessedafter the MS has first monitored the passive air interface, based onpassive interface monitoring criteria, likewise described below. Uponinterface reselection, the current active interface is preferablydeactivated (and becomes the passive interface) before activating thenew interface.

The passive interface monitoring criteria, in accordance with which theMS initiates passive interface monitoring, preferably includes all ofthe following conditions:

-   -   1. The network broadcasts an indication that neighbor cells on        the passive air interface are available; and    -   2. All cells received on the active interface have signal level        below a predefined broadcast threshold for a predetermined        period, preferably about 5 sec; and    -   3. There are fewer than a predetermined minimum number of cells,        preferably two cells, on the list of available neighbor cells        over the active interface for a predetermined period of time.        Alternatively, the MS begins passive interface monitoring if a        HPLMN search has been initiated, in accordance with GSM network        standards, or if the interface search timer, described        hereinabove, has expired while the MS is camped on a cell of the        less preferred interface.

Following the activation of passive interface monitoring, the interfacereselection criteria are assessed. If the criteria are met, interfacereselection follows. Otherwise, passive interface monitoring isdiscontinued, and begins again after a predetermined hysteresis period(assuming that the above monitoring criteria are still met).

FIG. 13 is a diagram that schematically illustrates combined reselectioncriteria for use in determining whether an air interface reselectionshould take place, in accordance with a preferred embodiment of thepresent invention. The combined reselection criteria preferably includesthe following parameters:

-   -   Interface priority (IP) to allow the prioritization of one air        interface or the other;    -   Interface reselection hysteresis (IRH), to prevent frequent        change between the interfaces;    -   Strong neighbors (SN) assessment, to take into account border        cell situations (FIG. 2) and try to change the active interface        before encountering abrupt end of coverage.

Some of these parameters are preferably broadcast by the network. Whenno such extended broadcast information is available, default values fromextended SIM 44 in MS 40 are preferably used. Otherwise, default valuesstored in the memory of MS 40 apply.

Preferably, interface reselection is based on measurements by MS 40 ofreceived RF power levels of strong neighbor cells. A “good interface” isone in which the strongest cell candidate (normally the serving cell) isreceived above a predetermined threshold. A “bad interface” is one inwhich all cells are received below the threshold, and there are lessthan the predetermined number (preferably two) of acceptable neighbors.Each of the active and passive air interfaces is classified on thisbasis as either “good” or “bad,” and the decision as to reselection ispreferably based on the following table:

TABLE 1 Combined Reselection Criteria based on Strong Neighbors ActivePassive Case Interface Interface Action 1 Bad Bad None 2 Bad GoodReselect interface 3 Good Good Reselect interface only if the passivemode has an advantage (i.e. a higher priority or HPLMN) 4 Good Bad None

Alternatively or additionally, the decision regarding interfacereselection is based on comparing path-loss values of the strongestcells over both interfaces as a measure of their relative quality. Inthis case, the decision to change from an active CDMA interface toGSM/TDMA is preferably based on the C1 c path loss criterion definedhereinabove and on a comparable GSM path loss criterion C1 g.Reselection takes place if:C1g(new intfc)+IPg>C1c(current intfc)+IPc+IRHc

On the other hand, the decision to change from an active GSM/TDMAinterface to CDMA is taken if:C1c(new intfc)+IPc>C1g(current infc)+IPg+IRHg

In these inequalities, IPg and IPc represent user preference values forthe GSM/TDMA and CDMA interfaces, respectively, which are preferablystored in SIM 44. IRHg and IRHc are respective interface hysteresisfactors, as described hereinabove. C1 g(new intfc) is the GSM path losscriteria for the new air interface and C1 c(current intfc) is the CDMApath loss criteria for the current air interface. Likewise, C1 c(newintfc) is the CDMA path loss criteria for the new interface and C1g(current intfc) is the GSM path loss criteria for the currentinterface. The user preference parameters IPc and/or IPg are taken intoaccount only when this is allowed by the network (preferably asindicated by a broadcast parameter IP_USE). Otherwise a priorityparameter broadcast by the network is used. This approach has theadvantage of supporting interface reselection based on a predefinedinterface priority, which a network operator can vary so as to adjustthe distribution of MSs between the available interfaces. Optionally, aconstant mapping factor is added to the inequalities to compensate fordifferences between the path-loss ranges of GSM/TDMA and CDMA.

Alternatively, passive interface monitoring is not used, and there is noindication given as to the existence of a better interface. In thiscase, interface reselection takes place only upon losing coverage of thecurrent active interface. This approach has the disadvantage, however,that it gives no assurance that the MS will camp on the best availablecell. Furthermore, the MS becomes non-pageable for a period of severalseconds upon loss of coverage.

General Comments

Although preferred embodiments are described hereinabove with referenceto a particular hybrid GSM/CDMA system, it will be appreciated that theprinciples of the present invention may similarly be applied to effectcell selection and reselection in other hybrid communication systems, aswell. Moreover, although the preferred embodiments make reference tospecific TDMA- and CDMA-based air interfaces and communicationsstandards, those skilled in the art will appreciate that the methods andprinciples described hereinabove may also be used in conjunction withother methods of data encoding and signal modulation. Furthermore, theprinciples of the present invention, which are exemplified herein byreference to a hybrid system including two types of air interfaces (CDMAand TDMA), may be applied in a straightforward manner to hybrid systemsincluding three or more different air interface types. The scope of thepresent invention encompasses not only the complete systems andcommunications processes described hereinabove, but also variousinnovative elements of these systems and processes, as well ascombinations and sub-combinations thereof.

It will thus be appreciated that the preferred embodiments describedabove are cited by way of example, and that the present invention is notlimited to what has been particularly shown and described hereinabove.Rather, the scope of the present invention includes both combinationsand subcombinations of the various features described hereinabove, aswell as variations and modifications thereof which would occur topersons skilled in the art upon reading the foregoing description andwhich are not disclosed in the prior art.

APPENDIX A Cellular Communications Standards

The following publications are incorporated herein by reference, asdefining standards with which system 20 generally and MS 40 particularlypreferably comply. Standards listed hereinbelow are cited at theappropriate places in the present patent application.

-   1. TIA/EIA-95-B: Mobile station-Based Station Compatibility Standard    for Dual-Mode Spread Spectrum Systems.-   2. TIA/EIA IS 98-B, May 13, 1998: Recommended minimum performance    for dual-mode spread spectrum cellular mobile stations.-   3. ETS 300 504: Phase 2 Types of mobile stations (GSM 02.06).-   4. ETS 300 507: Phase 2 Service accessibility (GSM 02.11).-   5. ETS 300 509: Phase 2 Functions related to Subscriber Identity    Module—functional characteristics (GSM 02.17).-   6. ETS 300 535: Phase 2 Functions related to Mobile Station in idle    mode (GSM 03.22).-   7. ETS 300 556: Phase 2 Mobile radio interface signaling layer    3—General aspects (GSM 04.07).-   8. ETS 300 557: Phase 2 Mobile radio interface layer 3 specification    (GSM 04.08).-   9. ETS 300 574: Phase 2 Multiplexing and multiple access on the    radio path (GSM 05.02).-   10. ETS 300 577: Phase 2 Radio transmission and reception (GSM    05.05).-   11. ETS 300 578: Phase 2 Radio system link control (GSM 05.08).-   12. ETS 300 608: Phase 2 Specification of the SIM-ME interface (GSM    11.11).

APPENDIX B Definition of Terms and Abbreviations

This appendix lists and defines certain technical terms andabbreviations, which are used in the context of the present patentapplication and in the claims. Although the terms and abbreviations areexplained in the body of the disclosure or are otherwise familiar tothose skilled in the art, they are repeated here for the convenience ofthe reader:

The term PLMN (public land mobile network) refers to a cellular network.We distinguish between the home PLMN (HPLMN) and the visited PLMN(VPLMN), as are known in GSM cellular systems.

The serving cell is the cell the mobile station (MS) chooses to camp on.Neighbor cells are cells that are located inthe vicinity of the servingcell and are declared to be neighbors by the PLMN.

“Camping on” a cell refers to tuning the mobile station receiver to thebroadcast channels of the cell, maintaining certain cell parametersinthe mobile station memory, and monitoring the paging channel of thecell.

“Air interface” refers to a set of interfaces and protocols used toestablish cellular communications. We distinguish between the GMS (orTDMA) air interface and the CDMA air interface.

The term “GSM/CDMA System” refers to a cellular system supportingdual-air interface operation.

The term mode is used in the following context:

-   -   Idle Mode—The mode of operation wherein no network radio        resources are explicitly allocated, and the mobile station        attempts to choose and camp onthe most adequate cell and to        receive the common broadcast channels thereof.    -   Dedicated Mode—The mode of operation wherein the mobile station        is connected to the network or accessing the network to        establish a service connection. In dedicated mode, the mobile        station is transmitting and receiving on channels specifically        allocated by the network.

ABBREVIATIONS Ba list BCCH allocation list of cells BCCH BroadcastCommon Control Channel Bit/s Bits Per Second BS Base Station BSC BaseStation Controller BSS Base Station Subsystem BTS Base TransceiverStation CB Cell Broadcast CBC Cell Broadcast Center CBCH Cell BroadcastChannel CBE Cell Broadcast Entity CC Call Control CFA CDMA FrequencyAssignment CGI Cell Global Identity CM Call Management CRH CellReselection Hysteresis dB Decibel dBm dB milliwatt DRX Discontinuousreceive mode GPRS Global Packet Radio Service GSM Global System forMobile Communications HLR Home Location Register HPLMN Home PLMN IMSIInternational Mobile Subscriber Identity IS Interim Standard L1 Layer 1L2 Layer 2 L3 Layer 3 LA Location Area LU Location Update MCC MobileCountry Code ME Mobile Equipment MNC Mobile Network Code MM MobilityManagement MS Mobile Station MSC Mobile Switch Center NV_MEM NonVolatile Memory PAM Paging Access Manager PLMN Public Land MobileNetwork RIL Radio Interface Layer RPLMN Registered PLMN RR RadioResource management SACCH Slow Associated Control Channel SAP ServiceAccess Point SDCCH Stand-alone Dedicated Control Channel SMS ShortMessage Service SMSCB Short Message Service Cell Broadcast VLR VisitorsLocation Register VPLMN Visited PLMN

APPENDIX C RR Sublayer Interfaces, Primitives and State Variables

This appendix describes aspects of GSM/CDMA RR sublayer 55 of signalinglayer 3, as shown in FIGS. 3-5, in accordance with a preferredembodiment of the present invention. The description covers onlyservices associated with the idle mode of MS 40 and does not coverprimitives that are relevant only for dedicated mode, which are beyondthe scope of the present patent application.

C1. the MM-RR Service Interface

The RR Combiner process (RRCO 84) implements a standard interface(RR-SAP 90) between the RR and MM sublayers of layer 3. The interface isin accordance with the GSM 04.07, substantially without modification.

C2. The RRCO-RRX Service Interface

The primitives defined in this section are a superset of GSM 04.07. Thedefinition reuses the MM-RR pirmitives described hereinabove and addsnew primitives to support the internal communication between the RRCOand RRX processes. (RRX is used herein as a collective term for RRG 80and RRC 82.) To avoid redundancy int his appendix, please refer to thedefinition of the RR-MM Service Interface in the GSM standard fordetails of identical service primitives and parameters.

C2.1. RRCO-RRX Service Primitives

TABLE C-3 RRCO-RRX Service Primitives PRIMITIVES PARAMETERSRRX_ABORT_IND As RR_ABORT_IND RRX_ABORT_REQ As RR_ABORT_REQ RRX_ACT_INDAs RR_ACT_IND RRX_ACT_REQ RrxReselectionParameters,RrxReselectionOptions RRX_BCCH_INFO_IND As RR_BCCH_INFO_IND RRX_EST_CNFAs RR_EST_CNF RRX_EST_IND As RR_EST_IND RRX_EST_REQ As RR_EST_REQRRX_EXT_NBC_INFO_IND ExtNbcInfo, ExtModeInfp RRX_GO_IDLE_PASSIVE_REQ —RRX_HPLMN_REQ As RR_HPLMN_REQ RRX_IDLE_PASSIVE_CNF — RRX_INFO_REQRrxCampedOptions RRX_PLMN_LIST_CNF As RR_PLMN_LIST_CNF RRX_PLMN_LIST_REQ— RRX_REL_IND As RR_REL_IND RRX_SAMPLE_IND NbrCellSamples RRX_SAMPLE_REQSampleOptions, ExtNbcList, SleepDuration RRX_SLEEP_IND SleepStartTime,SleepDurationRRX_GO_IDLE_PASSIVE_REQ

A request from RRCO to RRX to change from the active to the passivestate.

RRX_IDLE_PASSIVE_CNF

A confirmation from RRX to RRCO of deactivation of the lower layers. bysending this primitive, RRX confirms it is ready to acceptRRX_SAMPLE_REQ.

RRX_SLEEP_IND

Indication from the active RRX to RRCO upon entering the sleep periodfor the paging group in discontinuous receive operation (or slotted modefor CDMA).

RRX_EXT_NBC_INFO_IND

This primitive is used by RRX to provide RRCO with broadcase informationon nieghbor cells of the other air interface.

RRX_SAMPLE_REQ

A request from RRCO to RRX to perform an elementary cell measurementoperation within a predefined time frame

RRX_SAMPLE_IND

In the passive state, this primitive is used to respond toRRX_SAMPLE_REQ. When the MS camps on a cell in idle mode, it is used toprovide unsolicited periodical reports of the reselection parameters forthe serving cell and strongest neighbor cells.

C2.2. The RRXO-RRX Primitive Parameters

TABLE C-4 RRCO-RRX Primitive Parameters PARAMETER NAME DESCRIPTIONExtModeInfo A structure containing interface-specific parameters:Interface preference parameters Interface reselection parametersExtNbcInfo A structure containing extended information concerningneighbor cells belonging to the other air interface andinterface-specific parameters: NBC_DESCR SYS_TIME (only for CDMA)NCC_PERMITTED (only for GSM/TDMA) ExtNbcList A structure containing alist of neighbor cells for making measurements over the passiveinterface. NbrCellSamples Cell reselection parameters for one or moreneighbor cells. This is an array containing for each neighbor cell:NBC_DESCR C1 (pathloss criterion) RrxCampedOptions A structurecontaining the parameters of the serving cell. RrxReselectionOptions Astructure containing the cell selection parameters:IGNORE_FORBIDDEN_LAI_LIST RrxReselection A structure containing the cellselection Parameters parameters: Plmn_OrAny Ba_List SampleOptions Usedin the passive state of idle mode to define the type of measurement tomake (i.e. RF power, synchronization, cell information, extendedneighbor cell information). SleepDuration The duration of the sleepperiod in the camped state of the active RRX process, which can be usedto make one or more measurements using the other (passive) RRX.SleepStartTime The starting time of the sleep period of the active RRX.

1. A method used in a mobile wireless telecommunications system thatincludes base stations of a first type operating over a first airinterface and base stations of a second type operating over a second airinterface, the method performed by a mobile station camped on a cellassociated with a first base station of the first type, the method forreselection of a second base station of the second type, wherein thefirst and second air interfaces are distinct such that one is a TDMA airinterface and the other is a CDMA air interface, the method comprising:(a) receiving signals over the second air interface from the second basestation; (b) evaluating a characteristic of the signals; (c) responsiveto the characteristic, selecting the second base station in place of thefirst base station; and (d) camping on a cell associated with the secondbase station, wherein evaluating the characteristic comprises applying aCDMA path loss criterion to the signals.
 2. A method used in a mobilewireless telecommunications system that includes base stations of afirst type operating over a first air interface and base stations of asecond type operating over a second air interface, the method performedby a mobile station camped on a cell associated with a first basestation of the first type, the method for reselection of a second basestation of the second type, the method comprising: (a) receiving signalsover the second air interface from the second base station; (b)evaluating a characteristic of the signals; (c) responsive to thecharacteristic, selecting the second base station in place of the firstbase station; and (d) camping on a cell associated with the second basestation, wherein selecting the second base station in place of the firstbase station comprises using a single radio resource management protocollayer in the mobile station supporting both GSM/TDMA and CDMA operatingmodes and the radio resource management protocol layer comprisesparallel GSM and CDMA protocol sublayers and a combiner sublayer whichselects either the GSM or the CDMA operation mode.
 3. A method accordingto claim 2, wherein the combiner sublayer receives messages from amobility management protocol layer at a service access point inaccordance with GSM standards, and maps the messages to primitives whichit directs to the selected GSM or CDMA sublayer.
 4. A method used in amobile wireless telecommunications system that includes base stations ofa first type operating over a first air interface and base stations of asecond type operating over a second air interface, the method performedby a mobile station camped on a cell associated with a first basestation of the first type, the method for reselection of a second basestation of the second type, the method comprising: (a) receiving signalsover the second air interface from the second base station; (b)evaluating a characteristic of the signals; (c) responsive to thecharacteristic, selecting the second base station in place of the firstbase station; and (d) camping on a cell associated with the second basestation, wherein receiving the signals comprises regulating energyexpended by the mobile station in receiving the signals responsive to adesired level of energy consumption by the mobile station and whereinregulating the energy expended comprises setting a sampling rate atwhich to receive the signals responsive to the desired level of energyconsumption.
 5. A method used in a mobile wireless telecommunicationssystem that includes base stations of a first type operating over afirst air interface and base stations of a second type operating over asecond air interface, the method performed by a mobile station camped ona cell associated with a first base station of the first type, themethod for reselection of a second base station of the second type, themethod comprising: (a) receiving signals over the second air interfacefrom the second base station; (b) evaluating a characteristic of thesignals; (c) responsive to the characteristic, selecting the second basestation in place of the first base station; and (d) camping on a cellassociated with the second base station, wherein receiving the signalscomprises regulating energy expended by the mobile station in receivingthe signals responsive to a desired level of energy consumption by themobile station and wherein regulating the energy expended compriseschoosing a number of the base stations of the second type from which toreceive the signals responsive to the desired level of energyconsumption.
 6. A method used in a mobile wireless telecommunicationssystem that includes base stations of a first type operating over afirst air interface and base stations of a second type operating over asecond air interface, the method performed by a mobile station camped ona cell associated with a first base station of the first type, themethod for reselection of a second base station of the second type, themethod comprising: (a) receiving signals over the second air interfacefrom the second base station; (b) evaluating a characteristic of thesignals; (c) responsive to the characteristic, selecting the second basestation in place of the first base station; and (d) camping on a cellassociated with the second base station, wherein receiving the signalscomprises regulating energy expended by the mobile station in receivingthe signals responsive to a desired level of energy consumption by themobile station and wherein regulating the energy expended furthercomprises regulating the availability of the mobile station to receivethe signals responsive to a desired level of quality of service providedby the mobile station.
 7. A method used in a mobile wirelesstelecommunications system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the method performed by a mobilestation camped on a cell associated with a first base station of thefirst type, the method for reselection of a second base station of thesecond type, comprising: (a) receiving signals over the second airinterface from the second base station; (b) evaluating a characteristicof the signals; (c) responsive to the characteristic, selecting thesecond base station in place of the first base station; and (d) campingon a cell associated with the second base station, wherein evaluatingthe characteristic comprises comparing the signals received from thesecond base station to signals received over the first air interfacefrom the first base station and applying reselection criteria to thereceived signals so as to determine whether to select the second basestation and wherein applying the criteria comprises applying apredetermined hysteresis factor so as to prevent recurrent reselectionof the air interface.
 8. A mobile station for use in a mobile wirelesstelecommunications system that includes a first cell associated with afirst air interface and a second cell associated with a second airinterface, the mobile station comprising: (a) at least one radiotransceiver, which receives signals from the first and second cells overthe first and second air interfaces, respectively; and (b) controlcircuitry that processes the signal received from the second cell whilethe mobile station is camped in idle mode on the first cell and thatevaluates the second signal and, responsive thereto, directs the mobilestation to reselect and camp on the second cell, wherein the controlcircuitry is programmed to regulate energy expended by the mobilestation in receiving the signals responsive to a desired level of energyconsumption by the mobile station, and wherein the control circuitrysets a sampling rate at which to receive the signals responsive to thedesired level of energy consumption.
 9. A mobile station for use in amobile wireless telecommunications system that includes a first cellassociated with a first air interface and a second cell associated witha second air interface, the mobile station comprising: (a) at least oneradio transceiver, which receives signals from the first and secondcells over the first and second air interfaces, respectively; and (b)control circuitry that processes the signal received from the secondcell while the mobile station is camped in idle mode on the first celland that evaluates the second signal and, responsive thereto, directsthe mobile station to reselect and camp on the second cell, wherein thecontrol circuitry is programmed to regulate energy expended by themobile station in receiving the signals responsive to a desired level ofenergy consumption by the mobile station, and wherein the controlcircuitry chooses a number of cells from which to receive the signalsover the second air interface responsive to the desired level of energyconsumption.
 10. A mobile station for use in a mobile wirelesstelecommunications system that includes a first cell associated with afirst air interface and a second cell associated with a second airinterface, the mobile station comprising: (a) at least one radiotransceiver, which receives signals from the first and second cells overthe first and second air interfaces, respectively; and (b) controlcircuitry that processes the signal received from the second cell whilethe mobile station is camped in idle mode on the first cell and thatevaluates the second signal and, responsive thereto, directs the mobilestation to reselect and camp on the second cell, wherein the controlcircuitry is programmed to regulate energy expended by the mobilestation in receiving the signals responsive to a desired level of energyconsumption by the mobile station, and wherein the control circuitryfurther regulates the availability of the transceiver to receive thesignals responsive to a desired level of quality of service provided bythe mobile station.
 11. A mobile station in a mobile wirelesstelecommunications system, the system including a first cell associatedwith a first air interface and a second cell associated with a secondair interface, the mobile station comprising: (a) at least one radiotransceiver that receives signals from the first and second cells overthe first and second air interfaces, respectively; and (b) controlcircuitry that processes the signal received from the second cell whilethe mobile station is camped in idle mode on the first cell and thatevaluates the second signal and, responsive thereto, directs the mobilestation to reselect and camp on the second cell, wherein the controlcircuitry compares the signals received by the transceiver over thefirst and second air interfaces and applies reselection criteria to thecomparison so as to determine whether to select the second cell, andwherein the control circuitry applies a predetermined hysteresis factorto the comparison so as to prevent recurrent reselection of the airinterface.
 12. An apparatus used in a mobile wireless telecommunicationssystem that includes base stations of a first type operating over afirst air interface and base stations of a second type operating over asecond air interface, the apparatus camped on a cell associated with afirst base station of the first type, the apparatus performing a methodfor reselection of a second base station of the second type, wherein thefirst and second air interfaces are distinct such that one is a TDMA airinterface and the other is a CDMA air interface, the apparatuscomprising: (a) means for receiving signals over the second airinterface from the second base station; (b) means for evaluating acharacteristic of the signals; (c) responsive to the characteristic,means for selecting the second base station in place, of the first basestation; and (d) means for camping on a cell associated with the secondbase station, wherein evaluating the characteristic comprises applying aCDMA path loss criterion to the signals.
 13. A computer readable mediumcontaining executable instructions, which, when executed in a processingsystem, cause the system to perform a method used in a mobile wirelesstelecommunications system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the method performed by a mobilestation camped on a cell associated with a first base station of thefirst type, the method for reselection of a second base station of thesecond type, wherein the first and second air interfaces are distinctsuch that one is a TDMA air interface and the other is a CDMA airinterface, the method comprising: (a) receiving signals over the secondair interface from the second base station; (b) evaluating acharacteristic of the signals; (c) responsive to the characteristic,selecting the second base station in place of the first base station;and (d) camping on a cell associated with the second base station,wherein evaluating the characteristic comprises applying a CDMA pathloss criterion to the signals.
 14. An apparatus used in a mobilewireless telecommunications system that includes base stations of afirst type operating over a first air interface and base stations of asecond type operating over a second air interface, the apparatus campedon a cell associated with a first base station of the first type, theapparatus performing a method for reselection of a second base stationof the second type, the apparatus comprising: (a) means for receivingsignals over the second air interface from the second base station; (b)means for evaluating a characteristic of the signals; (c) responsive tothe characteristic, means for selecting the second base station in placeof the first base station; and (d) means for camping on a cellassociated with the second base station, wherein selecting the secondbase station in place of the first base station comprises using a singleradio resource management protocol layer in the mobile stationsupporting both GSM/TDMA and CDMA operating modes and the radio resourcemanagement protocol layer comprises, parallel GSM and CDMA protocolsublayers and a combiner sublayer which selects either the GSM or theCDMA operation mode.
 15. An apparatus according to claim 14, wherein thecombiner sublayer receives messages from a mobility management protocollayer at a service access point in accordance with GSM standards, andmaps the messages to primitives which it directs to the selected GSM orCDMA sublayer.
 16. A computer readable medium containing executableinstructions, which, when executed in a processing system, cause thesystem to perform a method used in a mobile wireless telecommunicationssystem that includes base stations of a first type operating over afirst air interface and base stations of a second type operating over asecond air interface, the method performed by a mobile station camped ona cell associated with a first base station of the first type, themethod for reselection of a second base station of the second type, themethod comprising: (a) receiving signals over the second air interfacefrom the second base station; (b) evaluating a characteristic of thesignals; (c) responsive to the characteristic, selecting the second basestation in place of the first base station; and (d) camping on a cellassociated with the second base station, wherein selecting the secondbase station in place of the first base station comprises using a singleradio resource management protocol layer in die mobile stationsupporting both GSM/TDMA and CDMA operating modes and the radio resourcemanagement protocol layer comprises parallel GSM and CDMA protocolsublayers and a combiner sublayer which selects either the GSM or theCDMA operation mode.
 17. The computer readable medium according to claim16, wherein the combiner sublayer receives messages from a mobilitymanagement protocol layer at a service access point in accordance withGSM standards, and maps the messages to primitives which it directs tothe selected GSM or GDMA sublayer.
 18. An apparatus used in a mobilewireless telecommunications system that includes base stations of afirst type operating over a first air interface and base stations of asecond type operating over a second air interface, the apparatus campedon a cell associated with a first base station of the first type, theapparatus performing a method for reselection of a second base stationof the second type, the apparatus comprising: (a) means for receivingsignals over the second air interface from the second base station; (b)means for evaluating a characteristic of the signals; (c) responsive tothe characteristic, means for selecting the second base station in placeof the first base station; and (d) means for camping on a cellassociated with the second base station, wherein receiving the signalscomprises regulating energy expended by the mobile station in receivingthe signals responsive to a desired level of energy consumption by themobile station and wherein regulating the energy expended comprisessetting a sampling rate at which to receive the signals responsive tothe desired level of energy consumption.
 19. A computer readable mediumcontaining executable instructions, which, when executed in a processingsystem, cause the system to perform a method used in a mobile wirelesstelecommunications - system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the method performed by a mobilestation camped on a cell associated with a first base station of thefirst type, the method for reselection of a second base station of thesecond type, the method comprising: (a) receiving signals over thesecond air interface from the second base station; (b) evaluating acharacteristic of the signals; (c) responsive to the characteristic,selecting the second base station in place of the first base station;and (d) camping on a cell associated with the second base station,wherein receiving the signals comprises regulating energy expended bythe mobile station in receiving die signals responsive to a desiredlevel of energy consumption by the mobile station and wherein regulatingthe energy expended comprises setting a sampling rate at which to,receive the signals responsive to the desired level of energyconsumption.
 20. An apparatus used in a mobile wirelesstelecommunications system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the apparatus camped on a cellassociated with a first base station of the first type, the apparatusperforming a-method for reselection of a second base station of thesecond type, the apparatus comprising: (a) means for receiving signalsover the second air interface from the second base station; (b) meansfor evaluating a characteristic of the signals; (c) responsive to thecharacteristic, means for selecting the second base station in place ofthe first base station; and (d) means for camping on a cell associatedwith the second base station, wherein receiving the signals comprisesregulating energy expended by the mobile station in receiving thesignals responsive to a desired level of energy consumption by themobile station and wherein regulating the energy expended compriseschoosing a number of the base stations of the second type from which toreceive the signals responsive to the desired level of energyconsumption.
 21. A computer readable medium containing executableinstructions, which, when executed in a processing system, cause thesystem to perform a method used in a mobile wireless telecommunicationssystem that includes base stations of a first type operating over afirst air interface, and base stations of a second type operating over asecond air interface, the method performed by a mobile station camped ona cell associated with a first base station of the first type, themethod for reselection of a second base station of the second type, themethod comprising: (a) receiving signals over the, second air interfacefrom the second base station; (b) evaluating a characteristic of thesignals; (c) responsive to the characteristic, selecting the second basestation in place of the first base station; and (d) camping on a cellassociated with the second base station, wherein receiving the signalscomprises regulating energy expended by the mobile station in receivingthe signals responsive to a desired level of energy consumption by themobile station and wherein regulating the energy expended compriseschoosing a number of the base stations of the second type from which toreceive the signals responsive to the desired level of energyconsumption.
 22. An apparatus used in a mobile wirelesstelecommunications system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the apparatus camped on a cellassociated with a first base station of the first type, the apparatusperforming a method for reselection of a second base station of thesecond type, the apparatus comprising: (a) means for receiving signalsover the second air interface from the second base station; (b) meansfor evaluating a characteristic of the signals; (c) means for responsiveto the characteristic, selecting the second base station in place of thefirst base station; and (d) means for camping on a cell associated withthe second base station, wherein receiving the signals comprisesregulating energy expended by the mobile station in receiving thesignals responsive to a desired level of energy consumption by themobile station and wherein regulating the energy expended furthercomprises regulating the availability of the mobile station to receivethe signals responsive to a desired level of quality of service providedby the mobile station.
 23. A computer readable medium containingexecutable instructions, which, when executed in a processing system,cause the system to perform a method used in a mobile wirelesstelecommunications system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the method performed by a mobilestation camped on a cell associated with a first base station of thefirst type, the method for reselection of a second base station of thesecond type, method comprising: (a) receiving signals over the secondair interface from the second base station; (b) evaluating acharacteristic of the signals; (c) responsive to the characteristic,selecting the second base station in place of the first base station;and (d) camping on a cell associated with the second base station,wherein receiving the signals comprises regulating energy expended bythe mobile station in receiving the signals responsive to a desiredlevel of energy consumption by the mobile station and wherein regulatingthe energy expended further comprises regulating the availability of themobile station to receive the signals responsive to a desired level ofquality of service provided by the mobile station.
 24. An apparatus usedin a mobile wireless telecommunications system that includes basestations of a first type operating over a first air interface and basestations of a second type operating over a second air interface, theapparatus camped on a cell associated with a first base station of thefirst type, the apparatus performing a method for reselection of asecond base station of the second type, the apparatus comprising: (a)means for receiving signals over the second air interface from thesecond base station; (b) means for evaluating a characteristic of thesignals; (c) responsive to the characteristic, means for selecting thesecond base station in place of the first base station; and (d) meansfor camping on a cell associated with the second base station, whereinevaluating the characteristic comprises comparing the signals receivedfrom the second base station to signals received over the first airinterface from the first base station and applying reselection criteriato the received signals so as to determine whether to select the secondbase station and wherein applying the criteria comprises applying apredetermined hysteresis factor so as to prevent recurrent reselectionof the air interface.
 25. A computer readable medium containingexecutable instructions, which, when executed in a processing system,cause the, system to perform a method used in a mobile wirelesstelecommunications, system that includes base stations of a first typeoperating over a first air interface and base stations of a second typeoperating over a second air interface, the method performed by a mobilestation camped on a cell associated with a first base station of thefirst type, the method for reselection of a second base station of thesecond type, comprising: (a) receiving signals over the second airinterface from the second base station; (b) evaluating a characteristicof the signals; (c) responsive to the characteristic, selecting thesecond base station in place of the first base station, and (d) campingon a cell associated with the second base station, wherein evaluatingthe characteristic comprises comparing the signals received from thesecond base station to signals received over the first air interfacefrom the first base station and applying reselection criteria to thereceived signals so as to determine whether to select the second basestation and wherein applying the criteria comprises applying apredetermined hysteresis factor so as to prevent recurrent reselectionof the air interface.